1. Field of the Invention
The present invention relates to an optical information apparatus capable of performing stable and highly accurate tracking control and tracking search for an optical information medium having a first region including tracks in which information is recorded in the form of convex and concave pits and a second region including tracks defined as convex and concave guide grooves.
2. Description of the Related Art
Optical information apparatuses (hereinafter also referred to as an "optical disk apparatus") for recording/reproducing signals on an information medium in the form of a rotating disk (hereinafter also referred to as an "optical disk") by converging or radiating a light beam from a light source (e.g., a semiconductor laser) are known. Such an optical disk apparatus performs the signal reproduction function by radiating a relatively weak light beam of a constant light amount on an optical disk so as to detect light reflected from the optical disk, where the intensity of the reflected light has been modulated by the optical disk. The recording of signals on the optical disk is performed by radiating a light beam whose light amount has been modulated in accordance with the signals to be recorded (hereinafter referred to as "recording signals") on an optical disk, thereby writing information on a film of recording material provided on the optical disk. Such a recording/reproduction technique is disclosed, for example, in Japanese Laid-open patent Publication No.52-80802.
An optical disk is typically fabricated by forming a film of material capable of optical recording/reproduction on the surface of a substrate having convex and concave tracks in a concentric arrangement by deposition or other techniques. FIG. 1A is a schematic perspective view showing the structure of an optical disk 1000 fabricated by such a method.
The optical disk 1000 includes two discrete doughnut-like regions along the radius direction of the disk, namely, a region 1, and a region 2, each including a plurality of tracks. FIGS. 1B and 1C are schematic magnified views of the optical disk 1000 at cross sections along the radius direction in the regions 2 and 1, respectively.
The region 2 shown in FIG. 1B, which includes a film of recording material formed thereon, defines a random access memory (RAM) region 2 where information can be recorded or reproduced in an optical manner. The RAM region 2 includes tracks 4 in the form of a continuous guide groove 5 (defining convex and concave portions) at a predetermined interval on the surface of a substrate 3. The tracks 4 have an optical depth substantially equal to .lambda./8 (where .lambda. represents the wavelength of an optical beam used during recording/reproduction). The tracks 4 in the RAM region 2 are typically formed at intervals of about 1.6 .mu.m.
The region 1 shown in FIG. 1C includes tracks 6 in the form of discontinuous grooves (defined by pits 7) formed on the substrate 3. The region 1 defines a read only memory (ROM) region 1 wherein information is previously recorded in the form of the pits 7. The tracks 6 in the ROM region 1 are also typically formed at intervals of about 1.6 .mu.m.
FIG. 2 is a block diagram showing the structure of an optical disk apparatus capable of searching for the tracks 4 and 6 on the optical disk 1000.
The structure shown in FIG. 2 includes a laser 11, a coupling lens 12, a polarizing beam splitter 13, a 1/4 wavelength plate 14, a total reflection mirror 15, a converging lens 16, an actuator 20, a detection lens 17, a cylindrical lens 18, and an optical detector 19, which are mounted on a conveyance base 10. The base 10 and the respective component elements mounted thereon are moved together along the radius direction (i.e., the "tracking direction) of the optical disk 1000 by a coarse motor 26.
Light generated by the laser 11 is colliminated by the coupling lens 12, led through the polarizing beam splitter 13 and the 1/4 wavelength plate 14, diverted by the total reflection mirror 15, and converged onto the optical disk 1000 by the converging lens 16. The light reflected from the optical disk 1000 goes back through the converging lens 16, the total reflection mirror 15, and the 1/4 wavelength plate 14, reflected by the polarizing beam splitter 13 so as to be led through the detection lens 17 and the cylindrical lens 18 and illuminate the optical detector 19 (having four discrete portions).
The converging lens 16 is attached to the actuator 20 via an elastic member, such as wire, so as to be movable along both a direction perpendicular to the surface of the optical disk 1000 (i.e., the focus direction) and the aforementioned tracking direction (i.e., the radius direction of the optical disk 1000). A focus coil and a tracking coil (neither of which is shown) are disposed on a fixed portion of the actuator 20. A permanent magnet (not shown) is disposed on a movable portion of the actuator 20 including the converging lens 16. The converging lens 16 travels along the focus direction due to electromagnetic force arising from a current flowing through the focus coil, and travels along the tracking direction due to electromagnetic force arising from a current flowing through the tracking coil.
FIG. 3 is a plan view showing the optical detector 19. The optical detector 19 includes four discrete light-sensitive portions A, B, C, and D disposed with respect to the tracking direction 8 and a longitudinal direction 9 of tracks, as shown in FIG. 3.
The current which the light-sensitive portion A outputs based on the amount of light received thereby is converted into a voltage by an I/V convertor 22 shown in FIG. 2. Similarly, the currents which the respective light-sensitive portions B, C, and D output based on their respectively received light amounts are converted into voltages by I/V convertors 23, 24, and 25 shown in FIG. 2.
First, focus control, which is performed for ensuring that the converged spot of the light beam is positioned on a recording surface of the optical disk 1000, will be described.
The difference between a signal obtained by adding the output signals of the I/V convertors 22 and 24 by means of an adder 31 and a signal obtained by adding the output signals of the I/V convertors 23 and 25 by means of an adder 32 is subjected to an operation by a differential amplifier 35, whereby a focus error signal (hereinafter referred to as "FES") indicating the state of light convergence on the information surface (i.e., the recording surface) of the optical disk 1000 is obtained. Such a detection method, generally called the astigmatic method, is disclosed in Japanese Laid-Open Patent Publication No.50-99561, for example.
The FES is supplied to the focus coil via a phase compensation filter 60, a switch 62, and a power amplifier 68. Thus, the converging lens 16 is controlled in accordance with the FES so that the converged spot of the light beam is positioned on the recording face of the optical disk 1000.
Next, tracking control, which is performed for positioning the light beam on the track center in the RAM region 2 where the tracks are formed as the guide grooves, will be described.
The difference between a signal obtained by adding the output signals of the I/V convertors 22 and 23 by means of an adder 33 and a signal obtained by adding the output signals of the I/V convertors 24 and 25 by means of an adder 34 is subjected to an operation by a differential amplifier 36, whereby a tracking error signal (hereinafter referred to as "TES") indicating the positional relationship between the light beam and a track on the optical disk 1000 is obtained. Specifically, a TES is detected based on the difference between a signal obtained by adding the outputs from the light-sensitive portions A and B of the optical detector 19 and a signal obtained by adding the outputs from the light-sensitive portions C and D of the optical detector 19. Such a detection method, generally called the push-pull method, is disclosed in Japanese Patent Publication for Opposition No.59-18771, for example. By the push-pull method, a tracking error is detected based on the facts that the reflected light of the light beam takes a symmetrical intensity distribution (along the right-left direction) when the light beam is on the track center or when the light beam is positioned between the adjacent tracks, and that the reflected light of the light beam takes a correspondingly asymmetrical intensity distribution (along the right-left direction) when the light beam is off the track center.
The TES is supplied to the tracking coil via a low-pass filter 43, a phase compensation filter 61, a switch 63, and a power amplifier 69. The output signal from the switch 63 is supplied to the coarse motor 26 via the adder 67 and the power amplifier 70. Thus, the converging lens 16 and the base 10 are tracking-controlled so as to locate the light beam on the track center.
Next, the tracking control within the ROM region 1 where the tracks are formed as a pit array will be described.
In the ROM region 1, the pits 7 are formed so as to define discontinuous grooves. In subregions of the ROM region 1 where the pits 7 are present, a TES can be obtained by the above-described method, as is possible in the RAM region 2. However, in any subregion of the ROM region 1 where no pits are present, a TES cannot be obtained by the above-described method. Consequently, the TES in the ROM region 1 is a signal which is modulated by the pits 7. Since the frequency of modulation due to the pits 7 is sufficiently higher than the tracking control band, a tracking can be fairly obtained by eliminating the high frequency components by means of the low-pass filter 43.
Next, a method for searching for a desired track will be described.
With reference to the structure shown in FIG. 2, a microcomputer 80 closes the switches 62 and 63 to perform focus control and tracking control so as to locate the light beam on a track on the optical disk 1000. On each track of the optical disk 1000, an address for identifying the position of the track is recorded in the form of pits. An adder 41 adds the output signals of the adders 33 and 34 and outputs a signal corresponding to the total amount of light obtained at the light-sensitive portions A, B, C, and D of the optical detector 19 to an address regenerator 42. The address regenerator 42 digitizes its input so as to read the address At, which is output to the microcomputer 80.
Once the address At of the desired track is input to the microcomputer 80, the microcomputer 80 obtains a current address A0 from the address regenerator 42 to calculate the number Nt (=At-A0) of tracks between the current track and the desired track. The microcomputer 80 also clears the count of a pulse counter 54. Thereafter, the microcomputer 80 opens the switch 63 to inactivate tracking control. Concurrently, the microcomputer 80 sets a number corresponding to the number Nt (of tracks between the current track and the desired track) at a D/A convertor 83. The output signal of the D/A convertor 83 is supplied to the coarse motor 26 via the adder 67 and the power amplifier 70, and the coarse motor 26 moves the base 10 toward the desired track based on the supplied signal.
The TES is input to a comparator 53 via the low-pass filter 43. As the base 10 moves toward the desired track, the comparator 53 generates a signal obtained by digitizing the TES into a signal having a high level and a low level. The digitized signal is supplied to the pulse counter 54.
FIG. 4A is a cross-sectional view of an optical disk 1000 along its radius direction, showing a track 4(6) on the substrate 3 of the optical disk. FIGS. 4B and 4C show the TES and the output signal of the comparator 53, respectively, obtained when the light beam travels across the track 4(6). As seen from FIG. 4C, the output signal of the comparator 53 goes high or low every time the light beam travels a distance equal to 1/2 of the track interval.
The pulse counter 54 counts the rising edge of the output signal of the comparator 53. The microcomputer 80 reads the count of the pulse counter 54, thereby detecting the number N1 of tracks the light beam has travelled across after the track search was begun. Thereafter, the microcomputer 80 calculates the value Nt-N1 and sets a value which is in accordance with the calculated value at the D/A convertor 83, thereby driving the coarse motor 26.
When the number Nt-N1 of tracks to be crossed before reaching the desired track becomes zero, the microcomputer 80 closes the switch 63 to activate track control. The microcomputer 80 reads the address of the track at which the light beam is located, and upon determining that the address is equal to the known address of the desired track, ends the search operation. If the microcomputer 80 determines that the address is not equal to the address of the desired track, the above-described search operation is repeated until the desired track is reached.
In the above-described conventional optical information (optical disk apparatus), the TES in the ROM region 1 becomes smaller in amplitude than the TES in the RAM region 2 because no TES is in fact obtained in the flat subregions of the ROM region 1 where no pits are present. Therefore, the tracking control for the ROM region 1 is relatively instable.
There may also be a further problem as follows: In the case where the track pitch is large, as shown in FIG. 5A, the converged spot 200 of the light beam is located so as to correspond to one row of track 6 (pit 7). However, some optical disks may have a smaller track pitch in order to meet the demand for increased recording density. By applying the tracking control technique to such a narrow-pitched optical disk by the above-mentioned push-pull method using a semiconductor laser of the same wavelength as a light source (since it is not practical to employ different semiconductor lasers as a light source depending on the kind of disks), the diameter of the converged spot 200 of the light beam becomes excessively large relative to the width of one row of track 6' (pit 7'), as shown in FIG. 5B. Consequently, an adjacent track 6' (pit 7') is inevitably radiated by the same light beam. This results in some reflection of light from the adjacent track 6' (pit 7'), which in turn may cause an error in the TES. As a result, it becomes difficult to perform stable tracking control and/or a tracking search process at a desired accuracy.